1. Field of the Invention
The present invention relates to a voltage converter. More specifically, the present invention relates to a switching type voltage converter widely used as a power source or a converter for an electronic equipment.
2. Description of the Background Art
FIG. 1 is a circuit diagram of a conventional voltage converter having a plurality of voltage outputs. Referring to FIG. 1, a DC voltage is applied from a DC power source 1 to a driving circuit 2. Driving circuit 2 turns on/off FETs 31 and 32 as switching elements alternately at a period of a prescribed frequency, by applying driving signals to the gates of FETs 31 and 32. FET 31 has its drain connected to one end of a primary coil 5 of a voltage transformer 4, while FET 32 has its drain connected to the other end of primary coil 5 of the voltage transformer 4. To a center tap of primary coil 5, a DC voltage is applied from DC power source 1. FETs 31 and 32 have their sources connected to a negative terminal of DC power source 1.
The voltage transformer 4 includes two secondary coils 61 and 62. Secondary coil 61 has one end connected to an anode of a rectifier diode D1, and the other end connected to an anode of a rectifier diode D2. Rectifier diodes D1 and D2 have their cathodes connected to one end of a choke coil L1, and the other end of choke coil L1 is connected to a voltage output terminal V1 OUT. A center tap of secondary coil 61 is connected to a ground terminal GND1, and between voltage output terminal V1 OUT and the ground terminal GND1, a smoothing capacitor C1 is connected.
Secondary coil 62 has one end connected to an anode of a rectifier diode D3, and the other end connected to an anode of a rectifier diode D4. Rectifier diodes D3 and D4 have their cathodes connected to one end of a choke coil L2, and the other end of choke coil L2 is connected to a voltage output terminal V2 OUT. A center tap of secondary coil 62 is connected to the ground terminal GND2, and between voltage output terminal V2 OUT and the ground terminal GND2, a smoothing capacitor C2 is connected.
The operation of the voltage converter shown in FIG. 1 will be described. In response to the driving signal from driving circuit 2, FETs 31 and 32 turn on/off alternately. When FET 31 turns on, current flows through the upper portion of primary coil 5 of voltage transformer 4, and as a result, an AC voltage is induced at upper portions of secondary coils 61 and 62. The AC voltage is rectified by rectifier diodes D1 and D3, made smooth by choke coil L1 and smoothing capacitor C1 and by choke coil L2 and smoothing capacitor C2, respectively, and DC voltage is output to voltage output terminals V1 OUT and V2 OUT. Thereafter, when FET 31 turns off and FET 32 turns on after a prescribed delay, current flows through the lower portion of primary coil 5. As a result, an AC voltage is induced at lower portions of secondary coils 61 and 62. The AC voltage is rectified by rectifier diodes D2 and D4, made smooth by choke coil L1 and smoothing capacitor C1 and by choke coil L2 and smoothing capacitor C2, respectively, and DC voltage is output from voltage output terminals V1 OUT and V2 OUT.
In the conventional voltage converter shown in FIG. 1, a plurality of DC voltages can be provided. However, respective DC voltages provided are fixed. However, it is sometimes necessary for some electronic equipments receiving DC voltages from such a voltage converter to receive different DC voltages. In such a case, it is necessary to provide a regulator at voltage output terminal V1 OUT or V2 OUT, in order to further convert the DC voltage to obtain a desired DC voltage. This requires an additional regulator, making device structure complicated, increasing the number of parts and hence manufacturing cost. In addition, efficiency in operation is decreased.